Multi-stack optical data storage medium and use of such medium

ABSTRACT

A multi-stack optical data storage medium ( 20 ) for rewritable recording using a focused radiation beam ( 19 ) entering through an entrance face ( 16 ) of the medium ( 20 ) during recording is described. The medium ( 20 ) comprises a substrate ( 1 ) with deposited on a side thereof a first stack ( 2 ) L 0  comprising a first phase-change type recording layer ( 6 ). The first recording stack ( 2 ) is present at a position most remote for the entrance face ( 16 ). At least one further recording stack ( 3 )L n , which comprises a further phase-change type recording layer ( 12 ), is present closer to the entrance face ( 16 ) than the first recording stack ( 2 ). A transparent spacer layer ( 9 ) is present between the recording stacks ( 2, 3 ). The further recording layer ( 12 ) is substantially of an alloy defined by the formula Ge x Sb y Te z  in atomic percentages, where 0&lt;x&lt;15, 50&lt;y&lt;80, 10&lt;z&lt;30 and x+y+z=100 with a thickness selected from the range of 4 to 12 nm and has at least one transparent crystallization promoting layer ( 11′, 13 ′) having a thickness smaller than 5 nm in contact with the further recording layer ( 12 ). A high optical transmission combined with a low crystallization time of the recording layer ( 12 ) of the L n  stack ( 3 ) is achieved making the medium ( 20 ) suitable for multi-stack high speed recording with a linear recording velocity of at least 12 m/s.

The invention relates to a multi-stack optical data storage medium forrewritable recording using a focused radiation beam entering through anentrance face of the medium during recording, comprising:

-   -   a substrate with deposited on a side thereof:    -   a first recording stack L₀ comprising a first phase-change type        recording layer, said first recording stack being present at a        position most remote from the entrance face,    -   at least one further recording stack L_(n), which comprises a        further phase-change type recording layer, being present closer        to the entrance face than the first recording stack,    -   a transparent spacer layer between the recording stacks, said        transparent spacer layer having a thickness larger than the        depth of focus of the focused radiation beam.

The invention also relates to the use of such an optical recordingmedium in high-speed applications.

An embodiment of an optical data storage medium of the type mentioned inthe opening paragraph is known from U.S. Pat. No. 6,190,750, filed byApplicants.

An optical data storage medium based on the phase-change principle isattractive, because it combines the possibilities of direct overwrite(DOW) and high storage density with easy compatibility with read-onlyoptical data storage systems. Data storage, in this context, includesdigital video-, digital audio- and software-data storage. Phase-changeoptical recording involves the formation of submicrometer-sizedamorphous recording marks in a crystalline recording layer using afocused relatively high power radiation beam, e.g. a focused laser-lightbeam. During recording of information, the medium is moved with respectto the focused laser-light beam that is modulated in accordance with theinformation to be recorded. Marks are formed when the high powerlaser-light beam melts the crystalline recording layer. When thelaser-light beam is switched off and/or subsequently moved relatively tothe recording layer, quenching of the molten marks takes place in therecording layer, leaving an amorphous information mark in the exposedareas of the recording layer that remains crystalline in the unexposedareas. Erasure of written amorphous marks is realized byrecrystallization through heating with the same laser at a lower powerlevel without melting the recording layer. The amorphous marks representthe data bits, which can be read, e.g. via the substrate, by arelatively low-power focused laser-light beam. Reflection differences ofthe amorphous marks with respect to the crystalline recording layerbring about a modulated laser-light beam which is subsequently convertedby a detector into a modulated photocurrent in accordance with therecorded information.

One of the most important requirements in phase-change optical recordingis a high data rate, which means that data can be written and rewrittenin the medium with a user data rate of at least 30-50 Mbits/s. A highdata rate is particularly required in high-density recording and highdata rate optical recording media, such as in disk-shaped CD-RW highspeed, DVD-RW, DVD+RW, DVD-RAM, DVR-red and DVR-blue, also calledBlu-ray Disk (BD), which respectively are abbreviations of the knownCompact Disk and the new generation high density Digital Versatile orVideo Disk+RW and −RAM, where RW and RAM refer to the rewritability ofsuch disks, and Digital Video Recording optical storage disks, where redand blue refer to the used laser wavelength. Such a high data raterequires the recording layer to have a high crystallization speed, i.e.a crystallization time of lower than 30 ns, during DOW. This alsoapplies to the recording layers of multi-stack versions of mentioneddisks. For DVD+RW, a user data bit rate of 33 Mbit/s is needed and forDVR-red 35 Mbit/s and for DVR-blue 50 Mbit/s (a CET of 35 ns) or evenhigher for higher speed versions. The complete erasure time (CET) isdefined as the minimum duration of an erasing pulse for completecrystallization of a written amorphous mark in a crystallineenvironment. The CET is generally measured with a static tester. TheAV-information stream determines the data rate for Audio/Video(AV)-applications but for computer-data applications no restrictions indata rate apply, i.e. the higher the better. Each of these data bitrates can be translated to a maximum CET which is influenced by severalparameters, e.g. thermal design of the recording stacks and therecording layer materials used.

To ensure that previously recorded amorphous marks can be recrystallizedduring DOW, the recording layer must have a proper crystallization speedto match the velocity of the medium relative to the laser-light beamduring DOW, i.e. the linear recording velocity. If the crystallizationspeed is not high enough the amorphous marks from the previousrecording, representing old data, cannot be completely erased, meaningrecrystallized, during DOW. On the other hand, when the crystallizationtime is short, amorphization becomes difficult because crystallitegrowth from the crystalline background is unavoidable. This results inrelatively small amorphous marks (low modulation) with irregular edges,causing a high jitter level. This limits the density and data rate ofthe disk. Therefore a stack with a relatively high cooling rate of therecording layer is highly desired.

Another important requirement for optical data storage media is the datastorage capacity. Applying multiple recording stacks may increase thiscapacity. Multi-stack designs may be represented by a symbol L_(n) inwhich n denotes 0 or a positive integer number. In this document, the“further” stack through which the radiation beam enters is called L_(n),while each deeper stack is represented by L_(n-1) . . . L₀. Deeper is tobe understood in terms of the direction of the incoming radiation beam.Note that in other documents this notation may be reversed and that L₀represents the stack closest to the entrance face and L_(n) the stackfarthest form the entrance face. Thus in case of a dual stack design twostacks L₀ and L₁ are present. L₁ has to be substantially transparent tothe radiation beam in order to make recording in the deepest “first”stack (L₀) possible. However, a L_(n) stack with layers that combines arelatively high transparency with still sufficient cooling and recordingproperties is difficult to obtain. In multi-stack optical phase-changerecording it is difficult to fulfil the high cooling rate requirementfor the further recording stack because of the absence of a transparentlayer with sufficient cooling capability in the further recordingstacks. Furthermore, the recording layer of the further recording stackitself may not be too thin because this may cause a high crystallizationtime of said recording layer.

Said known medium of U.S. Pat. No. 6,190,750 has a |IP₂IM₂I⁺|S|IP₁IM₁|structure for rewritable phase-change recording which has two metalreflective layers M₁ and M₂, which respectively are relatively thick,with a high optical reflection, and relatively thin, with a relativelyhigh optical transmission and substantial thermal conductivity. Irepresents a dielectric layer, I⁺ represents a further dielectric layer.P₁ and P₂ represent phase-change-recording layers, and S represents atransparent spacer layer. In this structure the laser-light beam entersfirst through the stack containing P₂. The metal layers not only serveas a reflective layer, but also as a heat sink to ensure rapid coolingfor quenching the amorphous phase during writing. The P₁ layer ispresent proximate a relatively thick metal mirror layer M₁ which causessubstantial cooling of the P₁ layer during recording while the P₂ layeris present proximate a relatively thin metal layer M₂ with limited heatsink properties. As already explained, the cooling behavior of arecording layer determines to a large extent the correct formation ofamorphous marks during recording. Sufficient heat sink action isrequired in order to ensure proper amorphous mark formation duringrecording.

In order to enhance the transmission of the L₁ stack, additional thin Mand I layers were introduced in the known medium from U.S. Pat. No.6,190,750. Stoichiometric or compound Ge—Sb—Te materials, e.g.Ge₂Sb₂Te₅, are used as the recording layer for the known recordingmedium, e.g. DVD-RAM disks. These stoichiometric compositions (region 31of FIG. 3) have a nucleation-dominated crystallization process. It meansthat the erasure of a written amorphous mark occurs by nucleation in themark and subsequent growth. A relatively high optical transmission ofthe recording layer can only be achieved when its thickness is lowerthan 15 nm. However, the data rate of the recording layer of the L₁stack is very low because the complete erasure time (CET) of theseGeSbTe compound materials is larger than 500 ns at a thickness of 8 nmor smaller and is shortened to 300 ns when is sandwiched between twothin SiC layer. Still these values are unacceptably high. For multirecording layer applications it is desirable that the recording layers,which are closest to the entrance face of the recording/readinglaser-light beam, have a relatively high optical transmission, hence arelatively low thickness, in order to allow writing and reading inunderlying recording layers combined with a low CET.

It is an object of the invention to provide a rewritable optical storagemedium of the kind described in the opening paragraph, having a furtherrecording layer with a relatively high optical transmissioncorresponding to a thickness of the further recording layer of lowerthan 12 nm, and a CET of maximum 35 ns, making it suitable for highspeed recording. High-speed recording is to be understood as recordingat a linear recording velocity, i.e. the velocity of the focusedradiation beam relatively to optical data storage medium, of at least 12m/s.

This object is achieved in accordance with the invention by an opticalstorage medium, which is characterized in that the further recordinglayer is substantially of an alloy defined by the formulaGe_(x)Sb_(y)Te_(z) in atomic percentages, where 0<x<15, 50<y<80, 10<z<30and x+y+z=100 with a thickness selected from the range of 4 to 12 nm andthat at least one transparent crystallization promoting layer having athickness smaller than 5 nm is present in contact with the furtherrecording layer.

These materials can be considered as the area surrounding and includingthe eutectic Sb₇₀Te₃₀ doped with Ge and have a growth-dominatedcrystallization process. It means that mark erasure occurs by directgrowth from the edge between the written amorphous mark and crystallinebackground. Nucleation within the written amorphous mark does not occurbefore this growth finished. The CET of these materials first decreasesrapidly with increasing the layer thickness and then increases againupon further increasing layer thickness. The shortest crystallizationtime is found at a thickness of about 10 nm.

In non-prepublished European patent application 02075496.6 CPNLO20099),filed by Applicants, a thickness range between 7 and 18 nm is proposedfor use in high data rate and high density optical recording systems,such as DVD+RW, DVR-red and -blue. These “eutectic” (growth type)materials are most suitable for high data rate and high densityrecording in both single and dual layer DVD and DVR, also called Blu RayDisk (13D), recording systems because the crystallization time decreaseswith the decrease of the recording amorphous mark size. “Eutectic”refers to eutectic Sb₇₀Te₃₀ and to substantially the area 32 as drawn inFIG. 3. For a higher recording density, dual layer or multi layer DVD,DVR systems are highly desired because the recording density can bedoubled or more. In the L₁ stack of a dual layer DVD/DVR disk, thethickness of the recording layer should be as thin as possible,preferably about 5 nm, to allow a high transmission. The shortest CET ofdoped “eutectic” Sb—Te (growth-type) recording materials is obtained atabout 10 nm. A short CET at a still thinner layer is required. It isproposed to use the eutectic Ge-doped SbTe as recording layer, which isin contact with a crystallization promoting layer and preferablysandwiched between two crystallization promoting layers such asnitrides, oxides of Si, Al and Hf. The use of crystallization promotinglayers is to enhance the crystallization rate of the recording layer,leading to a CET of about 30 ns at a thickness of about 5 nm and arecording-layer composition of Ge_(7.0)Sb_(76.4)Te_(16.6). The low-CETwindow is also improved (see FIG. 2).

The thickness dependence of the crystallization time of these“eutectic”—GeSbTe compositions may be understood as follows: the strongdecrease of the CET with the increase of the phase change layerthickness is a result of competition between the contributions of theinterface material and the bulk material. When the layer is relativelythin, the volume fraction of the material located at the interface islarge, which is often structurally very different from its bulk form,e.g. has more defects. With the increase of layer thickness, thefraction of the material that is in bulk form will increase, and above acertain thickness the bulk form will govern the behavior of thematerial. Apparently, the bulk materials have a more favorable growthspeed than the interface materials. The increase of the CET with thephase change layer thickness may be caused by the volume increase of thematerial. The crystallization process of a Ge—Sb—Te layer according toclaim 1 is growth-dominated. The volume of the material to becrystallized becomes important. The size of the crystallites istypically 10 nm. When the layer becomes thicker, a three-dimensionalgrowth is required, naturally a longer time needed. When the layer isthin, a two-dimensional growth is needed, which needs a shorter time.

However, when the recording layer becomes too thin, e.g. a few nm, theinterface plays a dominant role and may reduce the growth speed. Theimprovement of the interface results in a significant enhancement ofcrystallization speed.

Preferably, the transparent crystallization-promoting layer mainlycomprises a material selected from the group of nitrides, oxides of Si,Al and Hf and even more preferably a material selected from the group ofnitrides of Al and nitrides of Si. Nitrides of Al and Si, e.g. Si₃N₄,have a very good crystallization promoting behavior.

In a favorable embodiment of the optical storage medium according to theinvention the further recording layer has a thickness selected from therange of 4 to 8 nm. At the lower end of this range an opticaltransmission of the L₁-stack may be achieved which is larger than 50%.

In another favorable embodiment of the optical storage medium accordingto the invention the alloy has a composition defined by the formulaGe_(x)Sb_(y)Te_(z) in atomic percentages, where 5<x<8, 70<y<80, 15<z<20and x+y+z=100. A recording layer with a composition in this range hasproven to give excellent CET values as low as 25 ns at an optimalthickness of 10 nm.

In a further embodiment a metal reflective layer, semi-transparent forthe radiation beam, is present in the further recording stack. Thisreflective layer combines a relatively large heat conductivity with arelatively high optical transparency. The heat conductivity isadvantageous for the amorphous mark formation process, especially whenusing growth dominated recording layer materials according to theinvention. Especially Cu is preferred because it combines excellent heatconductivity with a relatively low chemical reactivity compared to forexample Ag. A high heat conductivity is advantageous for cooling therecording layer of the recording stack.

Preferably the recording layer of the further recording stack and one ortwo crystallization promoting layers in contact with the furtherrecording layer is sandwiched between further dielectric layers. Anoptimum thickness range for e.g. a dielectric layer between therecording layer and the metal reflective layer, is found between 3 and30 nm, preferably between 4 and 20 nm. This dielectric layer may be usedto tune the optical properties of the recording stack. When this layeris relatively thin, the thermal insulation between the recording layerand the metal reflective layer is decreased. As a result, the coolingrate of the recording layer is increased. Increasing the thickness ofthe dielectric layer will decrease the cooling rate.

An optimal thickness range for a further dielectric layer at a side ofthe recording stack closest to the entrance face is between 50 and 200nm. When the first dielectric layer has a thickness lower than 50 nm theoptical properties of the stack may be adversely affected. Thicknesseslarger than 200 nm may cause stresses in the layer and are moreexpensive to deposit.

In a special embodiment of the optical storage medium according to theinvention the first recording layer has the same composition as afurther recording layer. The first recording may be sandwiched betweendielectric layers similar to the dielectric layers of the furtherrecording layer. Crystallization promoting layers in contact with thefirst recording layer may be present but are optional. The thickness ofthe first recording layer may be thicker than 12 nm because it does notneed to have a high optical transparency.

The dielectric layers may be made of a mixture of ZnS and SiO₂, e.g.(ZnS)₈₀(SiO₂)₂₀. Alternatives are, e.g. SiO₂, TiO₂, ZnS, AlN and Ta₂O₅.Preferably the dielectric layers of the first recording stack comprisesa carbide, like SiC, WC, TaC, ZrC or TiC. These materials may give ahigher crystallization speed and better cyclability than a ZnS—SiO₂mixture.

For the metal reflective layer, metals such as Al, Ti, Au, Ni, Cu, Ag,Cr, Mo, W, and Ta and alloys of these metals, can be used. The substrateof the data storage medium is at least transparent for the laserwavelength, and is made, for example, of polycarbonate (PC), polymethylmethacrylate (EMMA), amorphous polyolefin or glass. Transparency of thesubstrate is only required when the laser-light beam enters therecording stacks via the entrance face of the substrate. In a typicalexample, the substrate is disk-shaped and has a diameter of 120 mm and athickness of 0.1, 0.6 or 1.2 mm.

The substrate may be opaque when the laser-light beam enters the stackvia the side opposite from the side of the substrate. In the latter casethe metal reflective layer of the stack is adjacent the substrate. Thisis also referred to as an inversed stack. An inversed stack is e.g. usedin the DVR disk.

The surface of the disk-shaped substrate on the side of the recordingstacks is, preferably, provided with a servotrack, which can be scannedoptically. This servotrack is often constituted by a spiral-shapedgroove and is formed in the substrate by means of a mould duringinjection molding or pressing. These grooves can be alternatively formedin a replication process in the synthetic resin of the spacer layer, forexample, a UV light-curable acrylate.

Optionally, the outermost layer of the stack is screened from theenvironment by means of a protective layer of, for example, UVlight-cured poly(meth)acrylate. The protective layer must be of goodoptical quality, i.e. substantially free from optical aberrations andsubstantially uniform in thickness, when the laser-light enters therecording stacks via the protective layer. In this case, the protectivelayer is transparent to the laser-light and is also called cover layer.For DVR disks this cover layer has a thickness of 0.1 mm.

Recording and erasing data in the recording layers of the recordingstacks may be achieved by using a short-wavelength laser, e.g. with awavelength of 660 nm or shorter (red to blue).

Both the metal reflective layer, and the dielectric layers can beprovided by evaporation or sputtering.

The phase-change recording layer can be applied to the substrate byvacuum deposition. Known vacuum deposition processes are evaporation(E-beam evaporation, resistant heat evaporation from a crucible),sputtering, low pressure Chemical Vapor Deposition (CVD), Ion Plating,Ion Beam Assisted Evaporation, Plasma enhanced CVD. Normal thermal CVDprocesses are not applicable because of too high reaction temperature.The layer thus deposited is amorphous and exhibits a low reflection. Inorder to constitute a suitable recording layer having a high reflection,this layer must first be completely crystallized, which is commonlyreferred to as initialization. For this purpose, the recording layer canbe heated in a furnace to a temperature above the crystallizationtemperature of the Ge—Sb—Te alloy, e.g. 180° C. A synthetic resinsubstrate, such as PC, can alternatively be heated by a speciallaser-light beam of sufficient power. This can be realized, e.g. in aspecial recorder, in which case the special laser-light beam scans themoving recording layer. The amorphous layer is then locally heated tothe temperature required for crystallizing the layer, without thesubstrate being subjected to a disadvantageous heat load.

High-density recording and erasing can be achieved by using ashort-wavelength laser, e.g. with a wavelength of 670 nm or shorter (redto blue).

The invention will be elucidated in greater detail by means of exemplaryembodiments and with reference to the accompanying drawings, in which:

FIG. 1 shows a schematic cross-sectional view of an optical storagemedium in accordance with the invention,

FIG. 2 shows the relation between CET (in ns) and the thickness d (inum) of the recording layer of the L₁ or L₀ stack for aGSb_(76.4)Te_(16.6) material with and without crystallization promotinglayer,

FIG. 3 shows a ternary phase diagram for Ge—Sb—Te.

In FIG. 1 the multi-stack optical data storage medium 20 for rewritablerecording is shown. A focused radiation beam 19, with a wavelength of670 nm, enters through an entrance face 16 of the medium 20 duringrecording. The medium has a substrate 1 made of PC having a diameter of120 mm and a thickness of 0.6 mm, with deposited on a side thereof afirst recording stack 2 comprising a first phase-change type recordinglayer 6. The first recording stack 2 is present at a position mostremote from the entrance face 16. A further recording stack 3, whichcomprises a further phase-change type recording layer 12, is presentcloser to the entrance face 16 than the first recording stack. Atransparent spacer layer 9 is present between the recording stacks 2, 3.The transparent spacer 9 layer has a thickness of 30 μm and may be madeof a UV curable resin known in the art provided by spin coating or aplastic sheet of e.g. PMMA or PC including a pressure sensitive adhesive(PSA) layer. The further recording layer 12 is substantially of an alloydefined by the formula Ge₇Sb_(76.4)Te_(16.6) in atomic percentages andhas a thickness of 5 nm. Two transparent crystallization promoting layer11′, 13′ having a thickness of 2 nm are present in contact with thefurther recording layer 12. The transparent crystallization promotinglayers 11′, 13′ mainly comprise the material Si₃N₄. A metal reflectivelayer 14, semi-transparent for the radiation beam 19, is present in thefurther recording stack 3 and mainly comprises the element Cu and has athickness of 6 nm.

Recording and reading is performed by means of a laser-light beam 19.Further dielectric layers 11 and 13 made of (ZnS)₈₀(SiO₂)₂₀ withthicknesses of 5 and 160 nm respectively are present. The thickness d ofthe recording layer 12 may be varied between 4 and 20 nm. Results of theeffect of this variation on the CET are shown in FIG. 2.

The first recording layer 6 is substantially of an alloy defined by theformula Ge₇Sb_(76.4)Te_(16.6) in atomic percentages and has a thicknessof 10 nm. Two optional transparent crystallization promoting layer 5′,7′ having a thickness of 2 nm are present in contact with the firstrecording layer 6. The transparent crystallization promoting layers 5′,7′ mainly comprise the material Si₃N₄. A second metal reflective layer 4is present in the first recording stack 3 and mainly comprises theelement Cu and has a thickness of 100 nm. Recording and reading isperformed by means of a laser-light beam 19. Further dielectric layers 5and 7 are present made of (ZnS)₈₀(SiO₂)₂₀ with thicknesses of 20 and 90nm respectively. The thickness d of the recording layer 6 may be variedbetween 4 and 20 nm. Results of the effect of this variation on the CETare shown in FIG. 2.

The layer structure of the L₁ stack 3 of the medium of FIG. 1 describedabove may be summarized as follows:

I(160)-N(2)-P(5)-N(2)-I(5)-M(6)-I(80), in which notation I represent adielectric layer 11 or 13, N a crystallization promoting layer 11′ or13′, P the recording layer 12, M the metal layer 14 while the numberbetween brackets represents the thickness in nm of each layer. With thisdesign the following optical transmission (T), reflection (R) andcontrast values of the L₁ stack 3 are obtained:

T_(c)=0.352 T_(a)=0.531 R_(c)=0.145 R_(a)=0.028, c and a denoting thephase, i.e. crystalline or amorphous, of the recording layer 12.Contrast=(R_(c)−R_(a))/R_(c)=0.807.

In another embodiment, not drawn, the structure of L₁ may be:

I(60)-N(2)-P(5)-N(2)-M(6)-I(80). Note that, compared to FIG. 1, thedielectric layer 11 between the metal layer 14 and the crystallizationpromoting layer 11′ has been deleted. This deletion may increase thecooling behavior of the stack 3 because the distance between therecording layer 12 and the metal layer 14 has decreased. The deletionfurther influences the optical properties of the stack in terms ofoptical transmission, reflection and contrast. An advantage is thatfewer layers are required, which is economical in manufacture. With thisdesign the following optical transmission, reflection and contrastvalues of the L₁ stack 3 are obtained:

T_(c)=0.460 T_(a)=0.624 R_(c)=0.144 R_(a)=0.056.Contrast=(R_(c)−R_(a))/R_(c)=0.611.

The phase-change recording layers 6 and 12 are applied to the substrateby vapor depositing or sputtering of a suitable target. The layers thusdeposited is amorphous and is initialized, i.e. crystallized, in aspecial recorder also called initializer. Further layers, with theexception of spacer layer 9 and a cover layer 15 are also provided byvapor depositing or sputtering of a suitable target. The radiation beam19 for recording, reproducing and erasing of information enters therecording layer 6 or 12 via the transparent cover layer 15. Thetransparent cover layer 15 has a thickness of 0.1 mm and is made of a UVcured resin provided by spin coating. The cover layer 15 may also beprovided by application of a plastic sheet including a pressuresensitive adhesive (PSA) layer.

In FIG. 2 the dependence of the CET in ns on the thickness d in nrm ofthe phase-change recording layer 6 or 12 for the compoundGe₇Sb_(76.4)Te_(16.6) is shown. Graph 2I represents the relation withoutcrystallization promoting layers and graph 22 represents the relationwhen the recording layer 6 or 12 is sandwiched between twocrystallization promoting layers made of Si₃N₄ and having a thickness of2 nm. From curve 21 it is clear that the CET has a minimum value at d=10nm. Further it is clear that by applying crystallization promotinglayers the CET stays below 35 ns even at a thickness of d=4 nm of therecording layer 6, 12.

In FIG. 3 the ternary phase diagram 30 has an area 32 which representsthe “eutectic” Ge_(x)Sb_(y)Te_(z)(x+y+z=100) materials that are used asthe recording layer for e.g. DVD+RW, DVR or BD disks and are far fromthe stoichiometric compositions in region 31. The materials withcompositions from area 32 can be considered as the eutectic Sb₇₀Te₃₀doped with Ge and have a growth-dominated crystallization process. Itmeans that mark erasure occurs by direct growth from the edge betweenthe written amorphous mark and crystalline background. Nucleation withinthe written amorphous mark does not occur before this growth finished.The CET of these materials first decreases rapidly with increasing thelayer thickness and then increases again upon further increasing layerthickness as shown in FIG. 2. The shortest crystallization time is foundat a thickness of about 10 nm. These eutectic (growth type) materialsare most suitable for high data rate and high density recording in bothsingle and dual layer DVD and DVR recording systems because thecrystallization time decreases with the decrease of the recordingamorphous mark size.

It should be noted that the above-mentioned embodiment illustratesrather than limits the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.The word “comprising” does not exclude the presence of elements or stepsother than those listed in a claim. The word “a” or “an” preceding anelement does not exclude the presence of a plurality of such elements.The mere fact that certain measures are recited in mutually differentdependent claims does not indicate that a combination of these measurescannot be used to advantage.

According to the invention a multi-stack optical data storage medium forrewritable recording using a focused radiation beam entering through anentrance face of the medium during recording is described. The mediumcomprises a substrate with deposited on a side thereof a first recordingstack L₀ comprising a first phase-change type recording layer. The firstrecording stack is present at a position most remote from the entranceface. At least one further recording stack L_(n), which comprises afurther phase-change type recording layer, is present closer to theentrance face than the first recording stack. A transparent spacer layeris present between the recording stacks. The further recording layer issubstantially of an alloy defined by the formula Ge_(x)Sb_(y)Te_(z) inatomic percentages, where 0<x<15, 50<y<80, 10<z<30 and x+y+z=100 with athickness selected from the range of 4 to 12 nm and has at least onetransparent crystallization promoting layer having a thickness smallerthan 5 nm in contact with the further recording layer. A high opticaltransmission combined with a low crystallization time of the recordinglayer of the L_(n) stack is achieved making the medium suitable formulti-stack high speed recording with a linear recording velocity of atleast 12 m/s.

1. A multi-stack optical data storage medium (20) for rewritablerecording using a focused radiation beam (19) entering through anentrance face (16) of the medium (20) during recording, comprising: asubstrate (1) with deposited on a side thereof: a first recording stack(2) L₀ comprising a first phase-change type recording layer (6), saidfirst recording stack (2) being present at a position most remote fromthe entrance face (16), at least one further recording stack (3) L_(n),which comprises a further phase-change type recording layer (12), beingpresent closer to the entrance face (16) than the first recording stack(2), a transparent spacer layer (9) between the recording stacks (2, 3),said transparent spacer (9) layer having a thickness larger than thedepth of focus of the focused laser-light beam (19), characterized inthat the further recording layer (12) is substantially of an alloydefined by the formula Ge_(x)Sb_(y)Te_(z) in atomic percentages, where0<x<15, 50<y<80, 10<z<30 and x+y+z=100 with a thickness selected fromthe range of 4 to 12 nm and that at least one transparentcrystallization promoting layer (11′, 13′) having a thickness smallerthan 5 nm is present in contact with the further recording layer (12).2. An optical storage medium (20) as claimed in claim 1, wherein thetransparent crystallization promoting layer (11′, 13′) mainly comprisesa material selected from the group of nitrides, oxides of Si, Al and Hf.3. An optical storage medium (20) as claimed in claim 2, wherein thetransparent crystallization promoting layer (11′, 13′) mainly comprisesa material selected from the group of nitrides of Al and nitrides of Si.4. An optical storage medium (20) as claimed in claim 2, wherein thefurther recording layer (12) has a thickness selected from the range of4 to 8 nm.
 5. An optical storage medium (20) as claimed claim 1, whereinthe alloy has a composition defined by the formula Ge_(x)Sb_(y)Te_(z) inatomic percentages, where 5<x<8, 70<y<80, 15<z<20 and x+y+z=100.
 6. Anoptical storage medium (20) as claimed in any one of claims 1, wherein ametal reflective layer (14), semi-transparent for the radiation beam(19), is present in the further recording stack (3).
 7. An opticalstorage medium (20) as claimed in claims 6, wherein the metal reflectivelayer (14) mainly comprises the element Cu.
 8. Use of an optical storagemedium (20) as claimed in claim 1, for high speed recording with arecording speed higher than 12 m/s.